The present invention relates to an apparatus and method for reproducing information from an optical information recording medium, using holography.
Currently, based on Blu-ray Disc (BD) standard using a blue-violet semiconductor laser, an optical disc having a recording density of as large as 50 GB may be commercialized even for consumer use. In the future, optical discs are desired to provide a large capacity comparable to the capacity of a HDD (hard disc drive) such as 100 GB to 1 TB. However, in order to realize such an optical disc of ultra-high density, a high-density architecture technology of a new system is required which is different from the conventional high-density technology including decreasing the wavelength and making the NA (numerical aperture) ratio of objective lens higher.
During researches relating to a next-generation storage technology having been made a holographic recording technology recording digital information using holography has been paid to attention. The holographic recording technology is defined such that a signal light having information of page data two-dimensionally modulated by a spatial light modulator is superimposed on a reference light within an optical information recording medium, and a pattern of interference fringes resulting at the superimposition causes a modulation in the refraction index of the optical recording medium to thereby record the information on the optical information recording medium.
When reproducing the information, the same reference light as used in the recording is irradiated to the optical information recording medium. At this time, a hologram recorded in the optical information recording medium functions like a holographic diffraction grating and so, the same light as the recorded signal light including phase information is reproduced as a diffracted light. The reproduced light is two-dimensionally detected fast using a light detector (photodetector) such as a CMOS or CCD. In this manner, the holographic recording technology allows two-dimensional information to be recorded on the optical light recording medium by once and further the recorded information to be reproduced. In addition, the holographic recording technology allows a plurality of two-dimensional data to be multiplex-recorded on one position of the optical information recording medium, thereby making it possible to achieve recording and reproduction of high-speed and large-capacity information.
As the holographic recording technology for example, JP-A-2004-272268 is published. This publication discloses a so-called angle-multiplex recording system, in which signal light fluxes are collected on an optical information recording medium by a lens, and at the same time a reference light of parallel fluxes is irradiated on the recording medium to interfere with each other thereby performing holographic recording on the recording medium, and further a different page data is multiplex-recorded with an incident angle of the reference light to the optical information recording medium being changed, by indicating the page data on a spatial light modulator. In addition, the JP-A-2004-272268 discloses collecting the signal light by the lens and arranging the aperture (spatial filter) to the beam waist of the collected light, so that the spacing between adjacent holograms may be made shorter to thereby increase the recording density and the storage capacity of the recording medium compared with the prior art angle-multiplex recording system.
For another holographic recording technology, for example, WO2004-102542 is published. This publication discloses an example using a shift multiplex system in which in one spatial light modulator, light from an inner pixel is used as a signal light, and light from an outer pixel is used as a reference light, both fluxes are collected on the optical information recording medium by the same lens, and the signal light and the reference light are subjected to interference around a focal plane of the lens to thereby record the hologram.
As an equalizing technology in hologram reproduction, for example, JP-A-2006-267539 is published. This publication discloses an art in which in order to effectively remove intersymbol interference which an attention pixel receives from the surrounding pixels filter coefficients are selected based on a binarized-data pattern by which a temporal determination is made.